Orthodontic treatment is effected by fixing small appliances often referred to as orthodontic brackets to a patient's teeth in an appropriate manner so as to correct malaligned teeth by applying an external force thereto through an archwire extending between the generally fixed orthodontic brackets. As to the structure of these orthodontic brackets, they are constructed of a body having small slots and are adhered directly on the labial or lingual sides of the teeth or welded to metal bands attached to the teeth by cementing or by some other method, as is generally known in the art.
As the form of use of the orthodontic bracket constructed as described above, a flexible archwire, which is curved so as to conform to a dental arch, is placed in an archwire slot in the bracket, and the tooth can be shifted over time by the restoring force of the archwire so that the teeth become well aligned. With the orthodontic bracket, a force can be applied to the tooth in a desired direction to correct the direction in which the tooth is shifted, rotated, tipped or torqued. This is effected by the three-dimensional inclination of the slot formed in the bracket body or by the desired bending of the archwire.
The conventional orthodontic bracket has tie wings, and a ligature wire or an elastomeric ligature ring is hooked on the tie wings so as to positively hold the archwire to prevent the archwire from coming out of the slot in the bracket. The dentition that should be treated is of malocclusion, in which the archwires are greatly deformed when engaged. Orthodontic treatment is accomplished by transmitting the restoring force of the archwires through brackets to the tooth roots. Generally, in an early period of the treatment, a thin, highly flexible round archwire is used, and an operation is required to loosen the ligature wire after ligating it such that the round wire freely slides within the slot (on a very low frictional basis). As treatment progresses, a thicker wire, a square ro rectangular wire, and a more highly rigid wire come to be used. When the alignment is nearly completed, they are held for a while to prevent relapse. At this time, there are cases where the tooth is strongly fastened by a ligature wire so that it practically does not shift.
In the orthodontic treatment as described above, different archwires of many varieties are used in succession during the course of the treatment. During patient visits to the clinic, it is necessary to remove the ligature wire and adjust the bend of the archwire or replace it. This operation requires much chair time and imparts discomfort to the patients.
The various problems due to the use of this type of a ligature wire can be overcome to some extent by using a lock-type orthodontic bracket. Namely, this lock-type orthodontic bracket has a structure which does not require the tie wire for ligation, and has a locking member incorporated into the bracket and capable of shifting for opening or closing the slot in the bracket. As the locking members, there are, for example, rotating-type and sliding-type locking members. Since the locking members are capable of shifting in open and/or closed positions whereby the retention of the archwire in the slot or disengagement of the archwire from the slot can be easily affected. One example of an orthodontic bracket as described above is shown in U.S. Pat. No. 6,168,428 issued Jan. 2, 2001 to Voudouris.
One such prior art bracket 10 is shown in FIGS. 1 and 2, and includes a body 25 having a bonding base 30 for attachment to a tooth. The body includes an occlusal-gingivally extending opening 35, a lingual vertical slot 40, a pair of laterally spaced gingival tie wings 50, and a pair of laterally spaced occlusal tie wings 45. The gingival 50 and occlusal tie wings 45 project from a labial surface 55 of the body 25, with the occlusal tie wings 45 defining one or more grooves 60 for receiving one or more arm members 65 of a locking shutter 70 through the occlusal tie wings 45. An archwire slot 95 is provided extending mesiodistally across the body 25 and between the gingival 50 and occlusal 45 tie wings at opposed mesial and distal sides of the body to accommodate an archwire 80. The locking shutter 70 is moveable between an open position in which placement and removal of the archwire is facilitated and a closed position in which placement and removal of the archwire is inhibited (as illustrated). In the closed position, a retaining portion 85 of the locking shutter 70 is friction fit into a retaining groove 90 formed adjacent to the archwire slot 95, in each of the gingival tie wings 50. During normal use, when the locking shutter 70 is in the closed position, as shown in FIG. 2A, the archwire 80 is pressed into, and seated entirely within the archwire slot 95. However, applicant has discovered that when an unexpected large external force is applied to the archwire 80, the archwire 80 can slip and enter the retaining groove 90, and become caught therein, as shown in FIG. 2B. FIGS. 2A and 2B are illustrated with the retaining groove 90 shown in exaggerated proportion for illustrative purposes.
In such a state, the archwire fails to shift smoothly inside the archwire slot where it belongs, resulting in a hindrance to and uneven tooth movement during the orthodontic treatment. To avoid such a state, it has been proposed to decrease the width W of the retaining groove, but if the width W is decreased, the functioning of the locking shutter, specifically with regards to its ability to press down wires ranging from the narrow round wires to full-size square wire, is impaired.
In addition, there has been a problem in that when an unexpectedly large external force is applied to the archwire, a tip portion of the locking shutter may become deformed as the archwire is twisted, thereby rendering the archwire retention unstable so as to pull out of the archwire slot. In the structure disclosed in U.S. Pat. No. 5,906,486, an arrangement is shown in which the position of the tip portion of the locking member is restricted in such a manner as to close both ends of the retaining groove. In such a structure, however, since the arrangement is provided so as to close both ends of the retaining groove, the width of the tip portion of the locking member must be inevitably made smaller than the length of the slot. Accordingly, there has been a drawback in that the tip portion of the locking member has insufficient rotational control since its length for holding the archwire is shortened. A second problem is that by making the locking shutter smaller, and therefore relatively weaker, there is less force to press against the archwire.
There is therefore a need in the art for an orthodontic bracket having improved archwire retention, and that addresses one or more of the aforementioned problems with the prior art.